1. Field of the Invention
The present invention relates to manufacturing methods for preparing positive temperature coefficient polymer compositions. More particularly, the present invention relates to a manufacturing method for preparing positive temperature coefficient polymer compositions of high loading and uniform distribution of filler.
2. Brief Description of the Prior Art
Positive temperature coefficient (PTC) polymer compositions or materials are well known in the art, and have found wide-spread use as PTC devices in numerous electrical and electronic apparatus. The primary function of the PTC devices incorporating these polymer compositions is to prevent overloading and/or overheating of an electronic circuit. PTC devices are capable of serving this function because of the characteristic of the PTC polymer compositions to abruptly increase their electrical (ohmic) resistance with a relatively small rise in temperature within a predetermined (tripping) temperature range. Thus the PTC devices act as a type of fuse that trips when the circuit in which it is incorporated in series draws more than a predetermined amperage, and allow normal operation again after the cause for increased amperage in the circuit has ceased to exist and the heat generated in the PTC device by the excess current is dissipated.
The positive temperature coefficient polymers incorporated in the PTC devices typically comprise a crystalline polymer matrix in which an electrically conductive material (filler) is uniformly distributed. A typical and frequently used polymer for the matrix is high density polyethylene (HDPE), and a typical and frequently used electrically conductive material for the filler is carbon black. The PTC polymer materials typically include a high loading of the filler, for example a typical composition may contain approximately 45 percent by weight of carbon black.
A detailed description of the principles of operation of PTC devices, materials and methods of manufacturing the same, can be found in published PCT application WO 97/06660 (published on Feb. 27, 1997) assigned to the same assignee as the present application, and in published PCT application WO 97/39461 (published on Oct. 23, 1997).
Several problems encountered in the prior art in connection with the manufacturing of the PTC polymers relate to or are caused by the high loading of the polymer matrix with the carbon black or other filler. Briefly summarized, the high loading renders it difficult to consistently achieve uniform distribution of the filler, and therefore difficult to achieve uniform resistivity. The high loading also results in relatively high wear and tear of the machinery utilized for manufacturing the PTC polymer material, causes frequent jamming and difficult clean-up after jamming of the machinery.
The present invention provides an improved process for manufacturing the PTC polymeric materials. The manufacturing process of the present invention results in more consistently uniform distribution of the filler in the polymer matrix, reduces wear and tear in the machinery, eases the task of cleaning-up the machinery, and allows for manufacturing PTC materials with higher loading of conductive filler than was consistently possible in the prior art.